Multilayer photoconductive device having adjacent layers of different spectral response

ABSTRACT

A photoconductive structure includes first layers of a photoconductive material having one spectral response region and second layers of a different photoconductive material having a different spectral response region laminated one on another either alternately or in a predetermined order at least two times, the thickness of each layer being such that light penetrates therethrough, i.e., less than 2 Mu . Low-resistivity photoconductive materials can be used to provide multilayer structures having a high sensitivity to a predetermined range of wavelengths of light, for example, to the red-light region and a wide spectral region.

United States Patent Inventors Yuji Kiuchi;

Hiroo Hori, Yokohama-shi; Shigeo Tsuji, Fujisawa-shi, Japan Appl. No.614,436 Filed Feb. 7, 1967 Patented Mar. 9, 1971 Assignee Tokyo ShibauraElectric Co., Ltd.,

Kawasaki-shi, Japan Feb. 14, 1966, Feb. 25, 1966 Japan 41/8334/66, 41/11,043, 41/1 1,044 and 41/1 1,045

Priority MULTILAYER PHOTOCONDUCTIVE DEVICE HAVING ADJACENT LAYERS OFDIFFERENT SPECTRAL RESPONSE 7 Claims, 7 Drawing Figs.

U.S. C1 313/65, 313/96, 313/112, 250/213 Int. Cl. ..H0lj 31/26, 1-101j39/14 Field ofSearch 313/65 (A),

[56] References Cited UNITED STATES PATENTS 2,869,010 1/1959 Gray2,594,740 4/1952 DeForest et al.

2,890,359 6/1959 l-leijne et a1 2,914,679 1 1/1959 Loebner 2,942,1206/1960 Kazan Primary Examiner-Roy Lake Assistant Examiner-V. LaFranchiAttorney-Stephen H. Frishauf ABSTRACT: A photoconductive structureincludes first layers of a photoconductive material having one spectralresponse region and second layers of a different photoconductivematerial having a different spectral response region laminated one onanother either alternately or in a predetermined order at least twotimes, the thickness of each layer being such that light penetratestherethrough, i.e., less than 2 p. Low-resistivity photoconductivematerials can be used to provide multilayer structures having a highsensitivity to a predetermined range of wavelengths of light, forexample, to

the red-light region and a wide spectral region.

PATENTEDHAR 9m 3.569.763

SHEET 1 [IF 3 PATENTEDMAR 91911 34569763 sum 2 0F 3 PATENTED'HAR slsn3,569,763

SHEET 3 UP 3 FIG. 5

RELATIVE SENSITIVITY PRIOR ARTB 5bo 4600 WAVELENGTH (mm) 7 5,100 O 80PRIOR ARTB' so 40 E 20 0 TIME(m sec) MUL'H'HJAYER PHGTOCONDUC DEVIQE HAGADJACENT LAYERS F DEFFEIRENT SPEC rouse The present invention relates tophotoconductive devices such as photoconductive targets for image pickuptubes, photoconductive cells and others and'the method for manufacturemanufacturing the same.

rhotoconductive targets for image pickup tubes are usually formed ofphotoconductive materials having dark resistivity of more than fl-crn.Currently available photoconductive materials which can satisfy theabove dark-resistivity requirement, however, are very few.Photoconductive targets made of conventional photoconductive materialssuch as antimony trisulfide and lead oxide exhibit limited spectralresponse since a photoconductive material is sensitive to radiations ina particular spectral region. A single photoconductive material,accordingly, can not cover all the desired wide spectral region ofwavelengths. By way of example, a target made of lead oxide, though ithas a high sensitivity to blue region of visible light, is notsumciently sensitive to red region, so that it can not be used forred-channel pickup tubes for color television broadcast.

To overcome the drawback as described above, it has been proposed toform a photoconductive target from a mixture of two or morephotoconductive materials respectively having different spectralresponse characteristics, for instance, a mixture containing lead oxidewhich is sensitive to blue region of visible light and lead sulfidewhich is sensitive to red region. A target formed of such a mixturepresents considerably improved spectral sensitivity distribution, butits electrical characteristics as the photoconductive target for thetelevision camera tube are deteriorated because of the incorporated leadsulfide whose dark resistivity is below 10 Q-cm, so that it can not bequalified for the practical use.

As a second alternative method it has also been proposed in US. Pat. No.2,687,484 to make a double layer structure of the target by laminatinglayers of different photoconductive materials respectively havingdifferent spectral response characteristics, for example, laminatingtogether a red-light sensitive antimony trisulfide layer 0.1 to 0.5 ,uthick and a blue-light sensitive amorphous selenium layer having athickness of 2 to 5 u. in such laminated structure, however, the firstlayer exposed to the incidence of light, the antimony trisulfide layerin the above example, should be made thin enough to permit incidentlight to penetrate therethrough sufficiently and reach the next layer,the amorphous selenium layer in the example, to the result that thefirst layer can not absorb incident light to a sufficient extent; thoughthe second layer sufficiently absorbs blue light, it is insensitive tothe residual red light unabsorbed by the first layer, and incident lightcan not be fully utilized. Further, in this structure photoconductivematerials of low resistivity can not be used, and hence reduced darkcurrent required for photoconductive devices can not be expected.

Accordingly it is an object of this invention to provide aphotoconductive device which has sufficient sensitivity over a requiredspectral region and which permits small dark current therethrough.

it is another object of this invention to provide a method formanufacturing such a device.

SUMMARY OF THE INVENTION According to this invention a novelphotoconductive device comprises a plurality of first layers of aphotoconductive material having one spectral response region and aplurality of second layers of a photoconductive material having adifferent spectral response region and each of the first and secondlayers being less than 2 p. in thickness so that incident lightpenetrates through and is gradually absorbed by successive layers. it isformed by alternately depositing by evaporation by of a mechanicalarrangement first layers of a photoconductive material having aparticular spectral response distribution and second layers of aphotoconductive material having a difi erent spectral responsedistribution from separate evaporating sources.

The invention is now described with reference to the accompanyingdrawings, in which:

FIG. l is a longitudinal section of an image pickup tube having aphotoconductive target embodying this invention;

FIG. 2 is an enlarged section partially illustrating the photoconductivetarget shown in FIG. 1;

FIG. 3 is an elevational section illustrating an apparatus formanufacturing the photoconductive target shown in FIG. 2;

FIGS. 4a and db illustrate a modification of the apparatus formanufacturing the target shown in FIG. 2, with FIG. 4a being elevationalsection of the apparatus and FIG. 4b being a plan view of a shutteringmember included in the apparatus;

FlG. 5 is a diagram comparing relative sensitivity versus wavelengthcharacteristic of the photoconductive device according to the inventionwith that of a conventional device; and

FIG. 6 is a diagram comparing residual signal of image or lag versustime characteristic of the photoconductive device according to theinvention with that of a conventional device.

Referring to the drawings, and particularly to FlG. l, the referencenumeral 10 generally designates an image pickup tube comprising anevacuated cylindrical envelope ll which coaxially encloses an electrongun 16 consisting of a cathode 12, a grid electrode 13, a firstaccelerating electrode 14, a second accelerating electrode 15 and a meshelectrode 17. A faceplate 18 closing the end of the envelope ll remotefrom the cathode 12 has its inner surface deposited with a trans parentconductive film 19 of, for instance, tin oxide. The conductive film 19is electrically connected to a signal electrode 20, and has in turndeposited on its side facing the electron gun 16 a multilayerphotoconductive structure, which embodies the invention and is generallyindicated at 32 and whose manufacture is described later in detail.

As is shown in more detail in FIG. 2, the multilayer photoconductivestructure 32 consists of juxtaposed pluralities of first and secondlayers 30 and 31, the lamination being, for example, obtained by firstvapor-depositing on the conductive film 19 a layer 30 of aphotoconductive material particularly sensitive to visible light inshort wavelength regions, for instance lead oxide, to have a thicknessof 0.8 ;1,, followed by the vapor deposition of a second layer 31 of adifferent photoconductive material particularly sensitive to visiblelight in long wavelength regions, for instance lead sulfide, to have athickness of 0.2 u, and repeating the alternate vapor-deposition ofthese two photoconductive materials until the thickness of thelamination reaches 10 ,u. The laminated structure 32 may have athickness ranging from 3 to 20 t and the thickness of individual layersis preferably less than 2 p..

In the operation of the image pickup tube 10, an electron beam 34 fromthe electron gun 16 is accelerated by accelerating electrodes 14 and 15and focused and deflected by focusing and deflecting coils not shown toscan the photoconductive layer 32 constituting the target. in theabsence of light falling on the target 32, its surface scanned by theelectron beam 34 is maintained roughly at the same potential as thecathode. When light reaches the target 32, the target is excited so asto reduce its resistivity depending upon the intensity of the incidentlight to produce corresponding electrical signals from the electron beamscanning the target.

The method of producing the above-mentioned multilayer photoconductivetarget is now described with reference to FIG. 3, which illustrates apreferred construction of the apparatus for manufacturing the multilayerphotoconductive target.

The apparatus comprises a bell jar 4t) gastight sealedly attached on theperipheral annular margin 42 of a support member all through a rubberpacking 43 to form a gastight vessel generally designated at 56. Withinthe gastight vessel 56 is disposed a coaxial d'mlike rotary substratesupport member 46 secured on a vertical shaft 65 sealedly and rotatablyextending through the support member 41 to the outside of the vessel 56and coupled to a rotor of drive means such as motor 44. The disclikerotary substrate support member 46, in this embodiment, is formed withcircular apertures 47 of the same diameter and symmetrical with respectto the axis of the shaft 45, the center of the apertures 47 being, forinstance, 8 cm. distant from the center of the disclike support member46. Over each of the apertures 47 is placed a substrate 48 constitutingthe faceplate of the image pickup tube. Above the substrates 48 isarranged a substrate heating means 49, for instance, consisting of anichrome wire.

Directly beneath the substrates 48 are disposed a corresponding numberof evaporation means, in this embodiment two of such means; one 50 madeof a platinum container containing lead oxide and the other 51 made of aquartz container containing lead sulfide and surrounded by a heatingcoil 52. The platinum container 50 is heated by applying current viaconducting leads 54 directly. The platinum container 50 and the heatingcoil 52 are electrically connected to associated conducting leads 54which are gastight sealedly brought out of the support member 41 andinsulated therefrom by insulating means such as ceramic ring members 53.Each of the evaporation means 55 is made of, for instance, glass toprevent evaporated photoconductive material from being scattered andoccupying all the space within the gastight vessel.

In operation, the container 50 is filled with 320 mg; of pulverized leadoxide while the container 51 is filled with 80 mg. of pulverized leadsulfide. Then the substrates 48 are heated to a temperature of from 90to 200 C., for instance, 150 C. by the heater 49. Thereafter the insideof the gastight vessel 56 is subjected to oxygen atmosphere at a lowpressure of from 2 X 10 to 3 X l mmHg, for instance, X mmHg. The mostsuitable value of said pressure is determined by an experiment. Then thecontainers 50 and 51 are heated to about 900 C. and to about 700 C.,respectively. After lead oxide and lead sulfide have been completelymelted by heating, the substrate support member 46 is rotated at a speedof, for instance, l5 r.p.m. With the rotation of the substrate supportmember 46, the substrates 48 placed over the support member 46 isalternately deposited with lead oxide layers, each layer being 0.8 p. inthickness and lead sulfide layers, each layer being 0.2 u in thickness.For instance a multilayer photoconductive structure having a thicknessof 10 u may be obtained if the substrate support member 46 is rotated 10times before it is stopped. Thus the thickness of one layer and that ofthe multilayer structure may be determined by specifying the speed andthe number of revolutions of. the substrate support member andevaporating conditions.

As in the previous apparatus, the apparatus generally indicated at 61 inFIG. 4a comprises a bell jar 64 secured on a support member 62 through arubber packing 63 making gastight seal of the apparatus. Within thegastight vessel 61 is disposed a coaxial stationary substrate supportmeans 68 whose top of disclike member is formed with a concentriccircular aperture 69 over which is placed a substrate 83 constitutingthe face plate of the image pickup tube.

Beneath and concentric with the circular aperture 69 is also disposed adisclike shutter member 67 in which is cut a sectorishaped notch asshown in FIG. 4b and which is sealedly and rotatably extending throughthe support member 62 to the outside of the gastight vessel 61 andcoupled to a rotor of a motor 65. As in the previous apparatus asubstrate heater 70 is arranged above the substrate 83. Beneath and inthe neighborhood of the shutter member 67 there are disposed a'pluralityof evaporation means: in this apparatus there are provided two of suchmeans spaced by a predetermined distance and are provided as in theprevious apparatus, one of which being platinum container 71 containinglead oxide and the other being quartz container 72 containing leadsulfide and surrounded by heating coil 73. The platinum container 71 iselectrically connected to associated conducting leads 8b and 81 whichare taken out of the gastight vessel 61 through insulating means toisolate them from the support member 62 such as ceramic ring members 78and 79, while the heating coil 73 is electrically connected toassociated conducting leads 76 and 77 which are similarly taken out ofthe gastight vessel 61 through ceramic ring members 74 and 75.

In operation, the platinum container is filled with 320 mg. ofpulverized lead oxide and the quartz container 72 with 80 mg. of leadsulfide. Then on the aperture 69 formed in the top of disclike member ofthe substrate support means 68 is placed a substrate 83, a transparentglass plate which is coated on its side facing the evaporation sourceswith a transparent conductor. Then the temperature of the substrate 83is elevated to from to 200 C., for instance, C. by the heater 70.Thereafter the gastight vessel 61 is evacuated and oxygen gas is filledtherein to provide low ambient pressure of from 2 X lo mml'ig. to 3 X101 mmI-Ig, for instance, 5 X 102 mmHg. Then the platinum container isheated to about 900 C. and the quartz container 72 is heated to about70020 C. After lead oxide and lead sulfide are completely melted byheating, the shutter member 67 is rotated at a speed of, for instance,15 rpm. by operating the motor 65. When the shutter member 67 is inrotation the substrate 83 is alternately deposited with lead oxidelayers, each layer being 0.8 p. thick and lead sulfide layers, eachlayer being 0.2 ,1. thick during alternate periods during which thenotch of the shuttermember 67 proceeds past the evaporating means 71 and72. As in the preceding embodiment, the multilayer photoconductivelamination structure may be made to have thickness of 10 ,u if theshutter member 67 is rotated 10 times before it is stopped. Also, it maybe made to have a desired thickness by suitably specifying the speed andthe number of revolutions of the shutter member and evaporatingconditions, the size of the notch in the shutter member and otheroperating conditions.

By fixing speeds of evaporation of individual photoconductive materialsand period of evaporation, the thickness of each layer of multilayerstructure may be made constant and reproducibility of photoconductivecharacteristics is very good. Also, since individual photoconductivematerials are evaporated from different evaporating means, each layer ofthe multilayer structure may be made to have a desired thickness byappropriately adjusting the temperature of the evaporating means, whichpermits controlling photoconductive characteristics of thephotoconductive structure, such as spectral response and sensitivity.

As described in the foregoing, according to the invention it is possibleto obtain a multilayer photoconductive target by alternately laminatinglayers of lead oxide 0.8 p. thick and sensitive to short wavelengthlights and layers of lead sulfide 0.2 p. thick and sensitive to longwavelengths. Such a multilayer structure is capable of efficientabsorption of visible light in a wide range of long wavelength region asrepresented by characteristic curve A in FIG. 5. As is seen from theFlG., the multilayer structure according to the invention is farsensitive to long wavelength regions without having reduced sensitivityto short wavelength regions as compared to the correspondingcharacteristic of a conventional lead oxide target as represented bycurve B in FIG. 5, which means improved performance and broadened fieldof use.

Further, as is seen from FIG. 6, the lag characteristic of thelead-oxide-lead-sulfide multilayer photoconductive structure accordingto the invention is greatly improved as represented by curve A in FIG. 6in comparison with the similar characteristic of a conventionallead-oxide-lead-sulfide mixture ghotoconductive structure as representedby curve B of FIG.

Heretofore, lead sulfide which has ideal sensitivity to long wavelengthregions, could not be applied for the photoelectric converting unit foran image pickup tube as its resistivity is essentially below 10 Q-cm.According to this invention lead sulfide can be used in combination withlead oxide having a high resistivity of over l0 (Lem as a multilayerstructure and time constant in the direction of thickness of theresultant multilayer structure can be made to be greater than 1 second.

The foregoing description has been concerned with the manufacture of aleadoxide-lead-sulfide multilayer photoconductive structure. Similareffects would result from multilayer structures of a combination oflead-oxide layers and lead-selenide layers, a combination of lead-oxidelayers and lead-telluride layers, and a combination of lead-oxide layersand layers of a mixture of lead telluride and lead sulfide or of leadsulfide and lead selenide, etc. I Y

The photoconductive materials which may be used in accordance with thisinvention include oxides, sulfides, selenides, tellurides of lead,antimony, arsenic, copper, silver, and cadmium, and suitable mixtures ofthese compounds.

In the foregoing description the lead-oxide layer and the lead-sulfidelayer are respectively formed from intrinsic substances. By doping leadoxide with an impurity element selected from a group consisting ofantimony, arsenic and bismuth, to form n-type photoconductivelayer,while doping lead sulfide with an impurity element selected from a groupconsisting of oxygen, silver, copper and thallium to form P- typephotoconductive layer, a multilayer photoconductive structure consistingof alternate NP-N-P....or PN-P- N....layer lamination may be obtained,which structure being effective in considerably reducing dark current.

The foregoing embodiments adapted the use of a photoconductive materialhaving a good sensitivity to light in shorter wavelength region and aphotoconductive material having a good sensitivity to light in longerwavelength region. In this point it is also possible to obtain aphotoconductive unit which is extremely sensitive to a desired spectralregion by laminating together layers of photoconductive material havinga relatively narrow spectral response region but good sensitivity andlayers of a different photoconductive material whose spectral responseregion lie near that of the first photoconductive material.

Further, in case of antimony triselenide whose resistivity is too low tobe used for television camera tubes, for instance, vidicon, byalternately laminating very thin layers of such low resistivephotoconductive material and also very thin insulating layers of, forinstance, silicon monoxide, more effective electric field can beestablished in the laminated structure, than the case of providing onlyone photoconductive layer having a greater thickness, so that producedfree carriers may effectively contribute to conduction. As theinsulating material may be used calcium fluoride and magnesium fluorideas well as silicon monoxide. 1

Furthermore, though the foregoing embodiments have been adapted forphotoconductive cells and image pickup tubes, similar effects may alsobe obtained when the multilayer structure is used for photoconductiveconverter units such as light amplifiers.

Moreover, the characteristics may be remarkably improved by depositingadditional photoconductive layer or layers on one side or on both sidesof the more than two material multilayer photoconductive structure. Byway of example, on the substrate may first be deposited a layer of amixture containing lead oxide and a trace of an oxide or a sulfide oftrivalent metallic element, for instance, antimony trisulfide, on whichis then deposited the layer with lead-oxide-lead-sulfide multilayerphotoconductive structure, on which is in turn deposited a thin layer ofa mixture containing lead oxide and a trace of an oxide on a sulfide ofa monovalent metallic element to improve sensitivity, darkcharacteristic and other various characteristics.

It will be understood that various changes and modifications may be madewithout departing from the scope of the invention as defined in theappended claims. It is intended, therefore, that all the mattercontained in the foregoing description and in the drawings are to beinterpreted as illustrative only and not as limitative of the invention.

We claim:

l. A multilayer photoconductive device having at least four layerscomprising:

first layers of a photoconductive material having a given spectralresponse characteristic; second layers of another photoconductivematerial having a different spectral response characteristic from thoseof said'first layers; and each of said first and second layers having athickness of less than 2 ;1., said first and second layers which are ofdifferent photoconductive material being alternately laminated on oneanother to form said device having at least four layers. 2. A multilayerphotoconductive device according to claim 1 comprising at least fivelayers.

3. A multilayer photoconductive device according to claim 1 comprisingat least seven layers.

4. A multilayer photoconductivedevice according to claim 1 for use as atarget for an image pickup tube wherein said target is particularlysensitive to red light and comprises at least two first layers of leadoxide and a corresponding number of second layers of lead sulfide, eachof said layers having a thickness of less than 2 ;1., said lead-oxideand lead-sulfide layers being alternately laminated on one another.

5. A The photoconductive target according to claim 4 wherein said firstlayers have a thickness of about 0.8 p. and

said second layers have a thickness of about 0.2 u.

6. The photoconductive target according to claim 4 wherein said firstlayers are of one type of conductivity and said second layers are of theopposite type of conductivity.

7. The photoconductive target according to claim 6 wherein said firstlayers contain at least one impurity element selected from antimony,arsenic and bismuth and said second layers contain at least one impurityelement selected from oxygen, silver, copper and thallium.

2. A multilayer photoconductive device according to claim 1 comprisingat least five layers.
 3. A multilayer photoconductive device accordingto claim 1 comprising at least seven layers.
 4. A multilayerphotoconductive device according to claim 1 for use as a target for animage pickup tube wherein said target is particularly sensitive to redlight and comprises at least two first layers of lead oxide and acorresponding number of second layers of lead sulfide, each of saidlayers having a thickness of less than 2 Mu , said lead-oxide andlead-sulfide layers being alternately laminated on one another.
 5. A Thephotoconductive target according to claim 4 wherein said first layershave a thickness of about 0.8 Mu and said second layers have a thicknessof about 0.2 Mu .
 6. The photoconductive target according to claim 4wherein said first layers are of one type of conductivity and saidsecond layers are of the opposite type of conductivity.
 7. Thephotoconductive target according to claim 6 wherein said first layerscontain at least one impurity element selected from antimony, arsenicand bismuth and said second layers contain at least one impurity elementselected from oxygen, silver, copper and thallium.